When a water droplet adheres to a surface of a sensor element of a humidity sensor due to dew condensation or water splash, it is difficult to detect correct humidity of a gas. For example, PLT 1 describes that when dew condensation occurs on a surface of a sensor element of a humidity sensor, as an electrostatic capacitance value varies extremely, dew condensation on the surface of the sensor element can be detected by monitoring the variation thereof.